No. 3.] VISUAL -CELLESTIN VERTEBRATES, 579 
but represents substance unblackened by osmic acid. After 
bleaching such material, and staining it with hematoxylin, 
I have obtained “in cross sections evidence that the outward 
projection of the corrugated outline represents a fiber circular 
in cross section and stainable. The central mass included by 
these peripheral structures seems quite homogeneous, as in 
fresh material. 
Frog rods treated by the same methods reveal a dark rim 
and a light line, but I have not been able to distinguish in 
them the corrugations. Such, however, have been reported 
by Hensen (67, Fig. 7) and others. 
Besides the ribbing of the surface and the underlying light 
spaces of the outer segments, I have seen no direct evidence 
of fibrillation in osmic material. That osmic acid fixation 
is in some ways very unfavorable for the demonstration of fine 
structural details, there can be no doubt. I have found no 
after-treatment by way of bleaching, etc., which satisfactorily 
restores the susceptibility to selective staining in the finer 
details. In other respects osmic fumes preserve very suc- 
cessfully the gross appearance of the fresh elements, in their 
almost optically homogeneous state. Whatever differentiation 
there is in the latter, is faithfully preserved in delicately fixed 
material. 
2. Vom Rath Material. 
Vom Rath’s picro-platino-osmo-acetic mixture, often used 
in the study of finer details of nerve cells, gives good preser- 
vation for the nuclear portions of the visual cells, but not 
for the distal parts. The outer segment of the rods becomes 
an opaque black, and as a rule contracts to one-half its normal 
length. The outer segment of the cones becomes much 
broken. In certain cases the longitudinal striation of the 
outer segment of the rods is plainly brought out (Pl. 4, 
Fig. 32, prs. dst.). Cross sections seem to indicate that the 
peripheral lines are not always confined to the periphery, but 
extend as clefts into the central mass (PI. 5, Fig. 40, bac.) 
